Rain repellent coating composition



Patented July 29, 1952 UNITED STATES PATENT- OFFICE" RAIN REPELLENTCOATING COMPOSITION Thomas P MayfWashington, D. C., and Oid DavisShreve, Chapel Hill, N. 'C.

No Drawing. Original application June 7, 1944,

Serial N0.'.539,178. Divided and this application April 7, 1945, SerialNo. 587,194

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757)- This application is a divisionof our abandoned application, Serial No. 539,178, filed June '7, 1944,which is a continuation in part of our abandoned application, Serial No.488,940, filed May 28, 1943.

Our invention relates to moisture-repellent coatings, and it isparticularly concerned with a rain-repellent coating composition andamethod of coating Windshields and other optical surfaces exposed to theweather with a moisturerepellent composition.

It is commonly appreciated that water when it strikes most surfacesspreads out into a more or less continuous film. This is due to the factthat most commonly encountered surfaces are somewhat hydrophilic, thatis to say, they are fairly readily wettable by water. The ability ofwater to wet surfaces becomes a distinctly annoying property when itwets transparent surfaces, such as Windshields of airplanes. There areother military applications where water films offer serious difiicultyas, for example, the viewing faces of submarine perisco'pes. Aircraft inflight often encounter rain and fog which cloud up windows andWindshields.

Water not only is troublesome in optical applications but also offersgreat difficulty in various electrical applications." Wherever highvoltages are used, parts are mounted on'stand-off insulators whichdepend for their efiectiveness upon givinga current a long, roughride'in creating a break-down. When high-voltage standoff insulators areused in aircraft,sudden changes in altitudes produce marked temperatureand humidity changes within the aircraft, condensation of, moisturetakes place on parts, and often spectacular break-downs occur. Elaborateprecautions have been taken against such contingencies bypressurizingequipment, heating equipment, and'separ'ating parts. I

From a consideration of those optical-and electrical problems so commonin aircraft, it became apparent to us that the same solution could beapplied in both cases because in both cases the action is a surfacephenomenon and continuous films of water cause the trouble. Manywaterproofing compositions are available for waterproofingpermeable-materials, but that is not the problem which it was'our objectto solve. Our problem is a peculiar mixture of electrical and chemicalin that both electrical and chemical surface phenomena are involved.- Aswe have stated, the problem is not to waterproof a surface,- for thesurfaces we propose'to treat are inherently-waterproof; glass and :theplastic material used as Windshields in automobiles and aircraft arecertainly waterproof in the sense inwhich the term is ordinarily used,as are also most of the ceramic insulators encountered in electricalapparatus. However, practically all surfaces of the classes we proposeto treat are wettable by water.

It is an object of our invention to provide a method of treating variouswettable but substantially water-impervious surfaces to make themnon-wettable by water.

,It is another object of our invention to provide certain novelcompositions for coating optical surfaces to make the surfacenon-wettable yet not adversely affect the optical propertiesof thematerial.

It is another object of our invention to provide a composition forcoating surfaces of electrical insulators to make them non-wettable.

' It is another object of our invention to provide a composition whichwhen applied to highvoltage insulating surfaces will withstand the peakvoltage to which the insulator will be subjected.

It is another object of our invention to provide a novel filmcomposition which will form a relatively hard, tack-free, and attractivesurface easily maintained by repeated application.

Other objects and advantages of our invention will in part be obviousand in part appear hereinafter;

Our invention comprises a novel film-forming composition possessing sucha degree of transparency, electrical resistance, non-wettability, anddurability that it can be used to put substantially invisible,long-lasting, non-wettable films on aircraft Windshields. Its electricalresistance is such that it can also be used-to coat electricalinsulators with a non-wettable filmto prevent their break-down bycondensation of moisture thereon. In the following description of ourinvention, we ,proposenot only todescribe how our film-formingcomposition is formulated but also how we think its electricalcharacteristics accentuate its value as a coating material for opticalsurfaces.

In their studies of surface phenomena physical chemists have adopted amethod of defining the wettability of a surface by a liquid. A drop ofthe liquid under study is allowed to'rest on the surface Whosewettability by the liquid is being measured and the contact angle of theliquid is determined. The contact angle. as we use the term throughoutthis specification is the angle vof inclination of a tangent to the dropat its point of contact with the surface, this In our opinion it is anessential characteristic.

of a rain-repellent film that it make the surface to which it is appliedform a contactanglegreater than 90 with the liquid which it is desiredto prevent from wetting the surface. If thecontact angle is less than90, a breeze of any strength will tend to make the droplet flattentransparency of the film is not important, the

composition and solvent can deviate from those given below. To achievegood results the solvent must. be selectedto give a reasonablyfast-drying time.- without causing separation of the solid out. If theangle is greater than 90; the breeze:

. film-forming compositions, according to our inwill make the dropletroll. Since it is the object of our invention to clear Windshields ofmoisture, obviously it isnecessary that our films cause Water toformwith them an angle greater than 90;

It is-our belief that the better'the-electricalinsulating properties ofthe film, the better. its:

non-wettable character. It is wellv known that aircraft in their swiftfiight will. pick up strong static charges particularly if theatmosphere through which they are flying contains particles of solid orliquid such as snow, hail, dust, or rain. To alimited extent such achargexon. the windshield, if' the wind'shield isa good insulator, Willbe helpful in that the windshield will not. permit.

a large leakage current to flow over it and, by, retaining a substantialcharge, will help: disperse drops finely by exploding. them astheystrike the surface.

It has been. found from experience. that a. continuous film'of waterover a transparent surface will interfere seriously with visibility,particularly if the film is in motion, for the ripples and waves in. thesurface of a moving film seriously impair visibility. It has also beenfound that large discrete drops on a windshield. also seriously impairvisibilty. When the drop. size is below about one millimeter, visibilityis not seriously affected.

From the above considerations, it. is clear that the ideal non-wettablefilm would be one that wouldmake water droplets form a contact. anglegreater than 90; it' would; retain enough static charge to keep dropletsbroken up finely, and it would be durable. In an aircraft orothervehiclethe wind would take care of the restby sweeping thesurface freeof the droplets-which, by virtue oftheir having high contact. angles,should roll off'quickly.

Although it is relatively easy to analyze. the problem of preparingnon-wettable films for optical and insulating surfaces andtosetupthe-properties'of theideal film, it is found: that the formula of ausable compositi-onis asevasive as a will-o-the-wisp. Film-formingmaterialshaving individual properties meeting onelorthe other of therequirements are common, butnon'e has the required combination of:desired; properties The composition of. our invention comprises threeessential ingredients to give. it the necessary film-forming. andelectrical properties, namely, a film-forming component, an adhesivewhich usually is also a plasticizer, and a hydrophobic material to givethe filmthe property of making high contact angles with water. Inpreparing the composition for. applicationto a: transparent surtests.

pane of glass mounted inclined at an angle of materials, and therelative proportions of the solid materials must also be selected togive a clear and uniformly smooth coating.

The-preparation of insulating and non-wettable vention and their usewill be clearly understood fromthe following; example:

Example ular weight. 5000 or greater) were dissolvedto.

form a clear solution in 85..parts butyl acetate.

It is possible. to vary the formulation of this coating composition byusing 3.0 to 6.0. parts nitrocellulose. (any nitrocellulose of. low orintermediate viscosity commonly usedv in lacquers, soluble in esters andester-alcohol mixtures), 10 to 14 parts glycerol. ester ofhyd'rog enatedrosin (or rosin, hydrogenated rosin or polyhydric alcohol ester of rosinor hydrogenated rosin) and 0.2 to 1.0 part of. polyi'sobutylene orparafiih. The amount of solvent can be variedto suit the formulator.However, about 85 parts, as mentioned above, gives a solution ofc-onvenient'viscosity; In all formulations for coating windshields andoptical surfaces; it is essential that the ingredients be compatible andremain compatible duringthe entire drying, time to avoid any' blushing.or separation of phases. on the treated surface. The above compositionwas tested, together with variations of it, in a rainmaking devicefor'standardization and in flight The rainmaking device consisted of a45, a blower and a spray; The blowerandf spray were arranged to send airand water against, the glass at a rate corresponding to. a rainfall -of4.0 inches per hour in a 40-mile' per hour gale. In all cases'it stoodupwell; inthe r'a'inmaking device it lasted 5=-6 hours in a con tinuousstorm of rain, striking it at the rate of 4.0 inches per hour in a40-mile, per hour gale". Since it is contemplated that the film isto beapplied to optical surfaces and" it wouldrequiregoodelectricalproperties for that purpose, it is hardly necessary tocite experimental data for its-use in coating insulators. It was foundthat these films'developed for" optical surfaces, in gen-' eral, were:excellent film formers for electrical insulators. The parts were coatedby dipping -or by brushing; For application to flat surfaces a squeegeemade of lintless material spreads'it on in. good fashion. Thecompositions all formed transparent, tough, durable films when permittedto dry for about an hour after application.

A few general properties of the compositions gelre brought. out aboveand. are summarized The ingredients recommended above are comnot affectthe surface to which it is applied, and

it must be fairly rapid drying.

The film-forming ingredient, nitrocellulose, should leave a durable filmafter the solvent evaporates.

The hydrophobic contact angle raising ingredient should be completelycompatible with the other materials used; for coating optical surfacesit must be transparent.

Films made according to the formulae given and suggested in the examplewere deposited on glass surfaces, and the surfaces, as indicated in theexample, repelled rain. Similar films were deposited on glass andplastic surfaces and soaked by immersion in water. Within 72 hours inevery case the films had soaked off. Similarly some matches and woodensurfaces were coated and immersed in water. In these cases, the filmssoaked loose in a matter of an hour or two, yet, in the case of thewooden surfaces, when exposed to rain the coated surfaces repelledwater. Apparently the surfaces are water repellent without beingwaterproof in the sense of being impermeable to water or water vapor. AsWe have pointed out in the early part of our description of ourinvention, we contemplate treating surfaces like glass and plastics orceramic insulating surfaces which are inherently impermeable to water.The permeabiliity of our films to water is, therefore, no disadvantagefor the films themselves show the desired degree of water repellency.

Since many apparently different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstoodthat we do not limit ourselves to the specific embodimentsthereof except as defined .in the appended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

Having described our invention what we claim as new and desire to secureby Letters Patent is:

l. A composition capable of drying to form a water-repellent filmconsisting essentially of from about 3 to about 6 parts ofnitrocellulose, from about 10 to about 14 parts of the glyceryl ester ofhydrogenated rosin and from about 0.2 to about 1 part of a hydrophobicmaterial selected from the group consisting of polyisobutylene of 5000average molecular weight, paraffin and ceresin in solution in butylacetate.

2. A composition capable of drying to form a water-repellent filmconsisting essentially of from about 3 to 6 parts of nitrocellulose,from about 10 to 14 parts of the glyceryl ester of hydrogenated rosinand from about 0.2 to about 1 part of parafiin in solution in butylacetate.

' THOMAS P. MAY.

OID DAVIS SHREVE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,737,187 Charch et a1. Nov. 26,1929 1,826,696 Charch et al Oct. 6, 1931 1,884,553 Bradley Oct. 25, 19321,940,462 Nollan Dec. 19, 1933 2,085,816 Meigs July 6, 1937 2,122,418Gladding et a1 July 5, 1938 2,181,609 Russell Nov. 28, 1939 2,305,497Rummelsburg Dec. 15, 1942 FOREIGN PATENTS Number Country Date 459,344Great Britain Jan. 6, 1937 OTHER REFERENCES Commercial Waxes, Bennett,1944, p. 71. The Chemistry and Technology of Waxes, Warth, 1947, pp.211, 212 and 287.

1. A COMPOSITION CAPABLE OF DRING TO FORM A WATER-REPELLENT FILMCONSISTING ESSENTIALLY OF FROM ABOUT 3 TO ABOUT 6 PARTS OFNITROCELLULOSE, FROM ABOUT 10 TO ABOUT 14 PARTS OF THE GLYCERYL ESTER OFHYDROGENATED ROSIN AND FROM ABOUT 0.2 TO ABOUT 1 PART OF A HYDROPHOBICMATERIAL SELECTED FROM THE GROUP CONSISTING OF POLYISOBUTYLENE OF 5000AVERAGE MOLECULAR WEIGHT, PARAFFIN AND CERESIN IN SOLUTION IN BUTYLACETATE.